Nut fasteners forming threaded joints with bolts on which they are assembled are subject to dynamic loads, stress reversals, shock, vibration and radical atmospheric fluctuations rendering them susceptible to undersireable loosening. When the joint design permits tightening of the nut fastener to high preload levels, the chances of nut loosening in service are significantly reduced. However, in many applications, the joint makeup, e.g. interposition of a soft gasket, accessory, etc. between the bolt and-nut is such that preload alone is inadequate and an assisting anti-loosening measure must be employed in order to maintain an installation standard. One such measure is the use of lock type fasteners.
Lock type fasteners in general comprise a broad family of mechanical fasteners each having a design capability to resist loosening after the fastener is assembled in its service application. They encompass both externally and internally threaded parts and are available commercially in a variety of different constructions.
Generally, lock type fasteners are characterized as falling into one of three basic categories, namely "free running", "prevailing torque", and "chemical reaction". The "free running" lock type nut fastener assembles freely until seated. It then gains resistance to loosening through the presence of an accessory device or through development of a tensile load in the externally threaded component as a result of tightening. The "prevailing torque" nut fastener typically has a self contained feature that creates frictional interference between the threads of the mating components. Finally, the "chemical reaction" fastener tends to adhesively bond mating threads through the application and reaction of chemicals deposited on or about the threads. They are primarily suited to applications intended to be permanent in that they have little, if any, re-use potential. Official regulation of the fastener nut industry is governed by the Industrial Fastener Institute (IFI) standard 100/107 or ANSI/ASME standard B18.2.2.
The common design principal of all lock type fasteners is to increase the break loose torque resistance between the nut and bolt. The major disadvantage of the free running type nut is that, when they break loose, resistance is overcome either through loss of fastener preload or ineffectiveness of the accessory device. Re-use resistance to fastener loosening becomes essentially zero and eventual disengagement of the mating parts becomes a possibility. Exemplary forms of free running lock type nut fasteners include jam nuts, cotter pins extending through a nut slot aligned with an aperture in the bolt shank; lock wire, lock washers, etc. The top lock form of lock nut is also free running until the top of the nut engages the bolt thread.
By contrast, prevailing torque type lock nut fasteners retain a continuing resistance to removal rotation even after fastener preload is fully dissipated so as to afford re-use suitability. Prevailing torque fasteners are typically of all metal that includes a thread modification of sorts or with an intervening non-metallic element added or fused to the threads. A form of prevailing torque lock nut is disclosed, for example, in U.S. Pat. No. 4,682,924 incorporated herein by reference.